The invention will be described with reference to the example of a rotary movement such as that of a printer disc, but it is applicable to all stepping movements in which the moving unit is liable to describe oscillations which can be substantially represented by a second-order system which is not critically damped.
If said time intervals between successive steps are not suitably chosen, the oscillating movement which follows the last pulse (and which may be due e.g. to the inertia of the moving unit and to the elasticity of the mechanical transmission parts) may continue for a relatively long time during which the position of the moving unit varies continually about the required position without stopping there. It is known to mitigate this disadvantage by means of a damping device; however, to be effective, this damping device must be able to damp appreciable energy; this increases its weight its bulk and its cost and subjects the motor to extra stresses.
German published patent application no. 2 042 903 describes an attempt to obtain this result by a particular method of defining the time interval between the last two pulses. However, it is clear that if the teaching of the application is followed, the desired result is not obtained, since it requires both the rotor inertia to be matched to a specific value and the total number of pulses to be sufficiently large with respect to the damping coefficient for the moving unit to have achieved steady state motion before the last pulse but one is applied thereto. These requirements are not specifically stated in the said specification, but they follow from the analysis of the problem.
The present invention provides a method of shifting a moving unit from a first position to a second position by means of a stepping motor driven by a finite sequence of pulses, said moving unit being liable to oscillate about its equilibrium point, the time intervals which separate said pulses being chosen as a function of the period and of the damping factor of the oscillation so that the last pulse is applied to the motor at an instant when the oscillation resulting from the set of preceding pulses brings the moving unit substantially to the required final position and reduces its speed substantially to zero, and wherein the last pulse but one of the sequence of pulses is applied to the motor at an instant when the point which represents the shift of the moving unit caused by said pulse and as shown in the phase-plane chart at a point centred on the equilibrium point resulting from said pulse is substantially located on the spiral which passes through the zero speed point and through the point of the x-axis equal to the amplitude of the last step.
Preferably the pulses other than the last and the first are applied at instants when the point which represents the shift of the moving unit, said shift being caused by each pulse and shown in the chart on the equilibrium point resulting from said pulse, is substantially located on said spiral.
The invention further provides a disc-type printer which includes a rotating disc which bears a set of characters and which can be driven by a stepping motor to place the required character in the required printing position, said disc being liable to oscillate about its equilibrium position; means for positioning the paper and making it advance; means for pressing the paper against the disc when said disc is substantially motionless; means which supply the motor with pulses; and means for triggering pulses at defined intervals; characterized in that the last pulse but one is applied to the motor in such a manner that the point on the phase plane chart representative of the motion of the moving parts after said last pulse but one, and with respect to the equilibrium position of the parts at that moment is situated on the spiral which passes through the zero speed point on the x-axis at a displacement corresponding to the last pulse and wherein the last pulse is applied to the motor substantially at the moment when the moving parts are in the desired final position.
The invention will be better understood from the description of an embodiment given hereinafter with reference to the accompanying drawing.